This application is directed to an improved AC susceptometer for measuring the properties of magnetic materials. This application is also directed to methodology for measuring the magnetic properties of materials.
AC susceptometers are widely used in measuring magnetic properties of magnetic materials in general and of superconducting materials in particular. The basic circuit of these systems is described in FIG. 1. It consists of a primary coil 20 coaxial with a pair of balanced coils 22,24, one of which, 24 contains the sample to be measured. A sinusoidal magnetic filed of a given frequency is created by the current output of an oscillator 26 energizing primary coil 20. This alternating field induces oscillations in the magnetization of the material, and as a result an off-balance voltage is induced in the coil pair. The material's response is measured by monitoring this signal at the driving frequency using a two-phase lock-in amplifier 28. The in-phase and the .pi./2 out-of-phase signals are used to derive the real part (.chi.') and imaginary part (.chi.") of the magnetic susceptibility of the material. In the case of materials such as superconductors, required to be maintained at a certain temperature, a cryostat 30 surrounds the coils 20, 22, 24.
This measurement technique is well suited for the characterization of material with a linear magnetic behavior, i.e., materials in which a pure sinusoidal field induces pure sinusoidal oscillations in the magnetization at the same frequency. However, under certain temperature and bias field conditions, materials can exhibit a nonlinear magnetic behavior; namely, a pure sinusoidal field can induce nonsinusoidal oscillations in the magnetization. Thus a nonsinusoidal voltage is induced across the sample coil and components of the voltage at harmonics of the driving frequency are generated. Conventional AC susceptometers measure the response of the material at the driving frequency, ignoring the harmonic components of the response.
In a previous application, of which we are coinventors entitled "Non-Contact Test of Materials for Superconductivity" Ser. No. 380,162 now U.S. Pat. No. 5,004,726, filed Jul. 14, 1989 a simple device for detecting superconductive transitions was described.